Water jet propulsion apparatus operate by utilizing the reaction forces resulting from propelling a mass in one direction thus creating an equal and opposite force in the other direction.
A high-pressure jet produces its thrust substantially in the nozzle section at the rear of the device. The impellers that produce the thrust are fine in pitch so that they are able to develop a pressure head, which in turn creates a large change in velocity as the water is forced through a rapidly reducing outlet. The water speed forward of the nozzle section in a water jet operating above the water line, is not the same as the water speed of the boat or craft. The water speed in the intake and impeller section is below boat speed, and so the change in velocity is calculated from the net change in velocity from the intake to the outlet of the nozzle, the greater change taking place in the latter.
Another form of water jet propulsion apparatus consists in a unit which delivers a considerable mass of water through an outlet nozzle but at a comparatively low pressure. Such devices are commonly known as a low pressure, high mass unit.
Water jet propulsion systems have attributes specific to the characteristic relating to the design of the unit. It is known that high pressure jet propulsion systems are particularly effective in shallow water operation. The shortcomings of a high pressure jet propulsion system however, relate generally to its slow to mid speed operation. A water jet requires high pressure in order to create a velocity change in the nozzle section sufficient to produce usable thrust. To achieve this, the known systems employ a fine pitched, pressure-inducing impeller or impellers, often followed by one or more stator sections, and then a reducing nozzle. The fine pitched impellers range from about 11-20 degrees, and thus have a reduced advance coefficient (ratio of boat speed to impeller tip speed). At slow impeller revolutions, they develop relatively low thrust.
A water jet propulsion system has a markedly reduced water speed forward of the nozzle section. Water diffuses into an intake section in front of the upstream Impeller, and as it does so, it slows down. This slowing down of the water as it passes through the body of the pump reduces losses through friction. The stators (water straightening vanes, placed downstream from the impellers) also represent a potential for unacceptable frictional losses if the water speed upstream from them is raised too high. The use of low advance coefficient impellers keeps the velocity low, but enables very high pressure to be produced in the nozzle section. This is where the greatest change in velocity takes place resulting in usable thrust. This locks a high-pressure jet system into having a configuration where a relatively low mass of water is accelerated to very high velocities in a nozzle section located downstream from all of these structures.
For a user who requires both good boat speed, but also slow speed control at low engine revolutions, the high pressure jet has limitations, as it expels a relatively low mass of water at low plume velocity. Where low impeller speeds and high propulsor thrusts are required, the high-speed jet is not a good substitute for a propeller system.
Considerable development has therefore been directed towards improving the efficiency of water jet propulsion units and in particular to provide a propulsion unit that can act as an effective high pressure low mass device and a low pressure high mass device.